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VIENNA A top aide to the chief of the U.N. nuclear agency has unexpectedly resigned, suggesting tensions among the organization's top leadership, diplomats said Friday.The move by IAEA Assistant Director General Rafael Mariano Grossi comes at a critical time for the International Atomic Energy Agency. It is the outside world's only window on Iran's nuclear program, which some nations fear is close to the ability to make atomic arms a goal Iran strenuously denies.IAEA inspectors monitor Tehran's known nuclear facilities including its expanding uranium enrichment program, which Tehran says is meant only to produce nuclear power and for other peaceful uses. But the United States, Israel, their allies and other nations fear the Islamic Republic could use the technology to make the core of a nuclear weapon.The agency also is trying to kick-start a probe of suspicions that Iran has secretly worked on developing nuclear weapons after more than five years of stagnation. Iran denies such work and says the allegations are based on falsified intelligence from Israel and the West. The two sides plan to resume talks on the issue in mid-May.Two diplomats demanded anonymity in exchange for speaking The Associated Press about the resignation because they were not authorized to discuss internal IAEA matters with reporters.One of them said Grossi told Amano he was quitting earlier this week after being told that his contract was not being extended. H Initial tests have come back negative for poisonous substance following a suspicious white powder found Thursday morning in the mailroom of the Naval Support facility in Arlington, Va.About 800 people were evacuated after the powdery substance was found in the mailroom of Building No. 12 at the support facility, the Navy said in a statement. All have since been let back in, Fox News has learned.Building 12 houses the mailroom and the offices of the chief of naval personnel, according to the official, who said the evacuations were a precautionary measure. The Arlington County Fire Department and hazardous-materials officials, as well as Navy personnel, are on the scene investigating.The incident comes just days after letters that tested positive for the poison ricin were mailed to President Obama and Sen. Roger Wicker, R-Miss, in Washington. The FBI arrested Mississippi resident Paul Kevin Curtis in connection with the letters Wednesday night, an FBI release said. Curtis is scheduled to appear in federal court Thursday and, if convicted, could face more than a decade behind bars.Fox News' Justin Fishel contributed to this report.